1. Field of the Invention
This invention relates to intrusion alarms. The invention relates more particularly to an improved, ultrasonic, pulse-echo method and apparatus for detecting the presence of an intruder in a protected space or for detecting certain other environmental changes.
2. Description of the Prior Art
Pulse-echo techniques are known for object detection wherein a pulse of acoustical energy is projected by a transducer and the occurrence of a pulse of reflected acoustical energy within a predetermined time interval is indicative of the presence of an object in an area being examined. Prior pulse-echo object detecting apparatus, however, have been relatively complex and costly and do not readily lend themselves to use as intrusion alarm detectors.
An improved pulse-echo method and apparatus for the detection of intrusions and other environmental changes is disclosed in my copending U.S. Patent Application Ser. No. 959,236 filed concurrently herewith and entitled IMPROVED INTRUSION DETECTION METHOD AND APPARATUS. In the method and apparatus disclosed in this copending application, pulses of acoustical energy are projected in a narrow beam at a reference surface and reflections of acoustical energy are detected. Any activities or environmental changes which alter a reflection from the reference surface are detected and an alarm is sounded.
As further disclosed in the copending application, the narrow beam of acoustical energy is projected at the reference surface and the range or distance of the reference surface from the apparatus is automatically determined and utilized as a parameter in sensing intrusions or other environmental changes. In general, the apparatus will automatically range on a dominant, larger, and substantially planar surface in an area to be protected, such as a wall surface.
Under certain circumstances, it is preferable to range on a specific target or object. For example, the particular arrangement of doors, windows and furnishings in an individual home may render it preferable to range on a particular reference object such as a bureau, a TV set, a chair, a door, a window, or the like. Although selection of a particular reference object can be accomplished with automatic ranging by the proper location of the pieces with respect to the detection apparatus, at times the reorientation of the object cannot be readily provided or the object cannot be separated sufficiently from proximity with a more dominating reference surface. It would be advantageous to enable the user of the intrusion detector to select a specific reference object without necessitating the orientation or rearrangement of objects.